Is there anybody out there?

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We can probably detect places in the galaxy that might harbour life, and we might be able to tell whether it's intelligent or not, but what are the odds that they're there at all?

50 years ago astrobiologist Frank Drake proposed an equation that now bears his name to estimate of the likelihood of discovering intelligent extraterrestrial life in the galaxy. Astrophysicist Duncan Forgan has been using computer modelling together with recent astronomical discoveries, to build upon Frank Drake's work to get a more accurate idea of whether there's anybody out there, as he explained to Graihagh Jackson...

Duncan - Well, the Drake equation is a series of seven or eight terms, depending on who you speak to. These terms get multiplied together and when those terms are multiplied together, you get a number, and that number tells you, if you look up at the night sky, that's how many communicating intelligent species there are.

Graihagh - Okay, so in this Drake equation, you start with how many stars are forming every year in our galaxy and then you subtract out all the stars that don't have any planets around them, right? Because you can't have life there, and then you subtract out all the planets that are too far or too close to the sun to support life, and so on and so on. You get the idea. Until finally, you come up with the number of planets, with intelligent life, that could transmit radio signals and be alive right now. So when you slot all the numbers into this equation, what'd you get?

Duncan - Typically, when I try and write down numbers in this equation I get the total number of intelligent civilizations to come out in quite small numbers, let's say ten civilizations in the milky at this point in time.

Graihagh - Ten. Does that include us or exclude us?

Duncan - Strictly speaking, that includes us, so you should take one off. It should be nine.

Graihagh - Nine? That's even worse!

Duncan - Yes. It's quite a tough challenge. Before we get into this, I should just point out that SETI scientists and astrobiologists don't use Drake's equation as a predictive tool. It's quite clear that it's very simple and kind of cuts out a lot of the nuance and the sophistication required to make an answer to that kind of question.

Graihagh - I'm wondering 50 years on, is there not a better way to make a more informed guess?

Duncan - Other ways you can look at this question would be to say, okay, we have statistical information on how stars form, we can say things in a similar vein about the masses of planets, we can say things about the likelihood of them existing a certain distance from the star, so you can build up a statistical picture of what the Milky Way looks like in terms of its star and planet population. And once you have that, then you can essentially build a model in which you can test different scenarios.

Graihagh - So what predictions have you been able to make?

Duncan - So when I first started doing this, I ran several different scenarios. Some were quite optimistic and some were quite pessimistic. And over the course of the Milky Way's existence, I showed that there would be somewhere between, you know, 300 and 30,000.

Graihagh - Not so many given how many stars and planets there are out there then.

Duncan - That's right. Of course, these are very speculative numbers though, because we're still stuck with the same problem that we just don't know how life forms on other worlds and we don't know really how that life becomes intelligent. So you're still saddled with these uncertainties but you can say given our uncertainties and making some sensible educated guesses, here are some nice sensible looking bounds in which we can put those numbers.

Even when you take a fairly optimistic view of the equation and you put in some quite generous values for some of the terms and if you get a number that's say tens of thousands, which sounds like a much bigger number. The Milky Way is very big, it's very big in space, it's about a hundred thousand light-years across and it is also very big in time, it's billions of years old. So we have this kind of dual quandary of how you get two civilizations to be close to each other in space and close to each other in time to appear about the same time. If one sends a radio signal, the other can see it before the other civilization goes defunct for whatever reason that might be.

Graihagh - And by Duncan's calculations, intelligent life that can transmit radio signals across the galaxy are likely to be alive for just one thousand years. A thousand years in a galaxy that is 13.2 billion years old with 300 billion stars and many, many more planets. When you take all of this into account, it seems pretty improbable that if there is anyone up there that will ever get the chance to talk.

Duncan - The probability might be low but that doesn't mean it's impossible, it just means that it's improbable. So even if we carry out a search knowing that we're likely to fail, actually failing in the search is quite important because again, I'm just a theorist with a computer making some assumptions. At the end of the day, if you want to do it scientifically, you have to go out there and test it. So, you've to make sure that you've got observations to back up your theories and if we made a null detection after a significant and deep search, that has a lot of information in it as well as the opposite case where did detect something. It will still tell us something quite fundamental and important about what it means to be human.

Graihagh - I was gonna' say what would that mean if we were the only sentient beings out there.

Duncan - It means that our existence is quite lonely and quite unique, and that human life in itself is even more precious than we might have thought originally.

Graihagh - And in some senses I guess then that would also mean that intelligent life of the whole galaxy rests on our shoulders.

Duncan - Well, in that sense, the stewardship of the Earth is not the only thing we are stewards of. We become stewards of a much larger volume of the universe and we have to be very careful and look after it if that is the case. So again, that has very strong implications for how we should behave as a civilization.